EMR EMISSIONS FROM REMOTE AREA POWER SUPPLY EQUIPMENT

Transcription

1 Radiation Protection in Australasia Vol. 22, No. 2 Refereed Paper EMR EMISSIONS FROM REMOTE AREA POWER SUPPLY EQUIPMENT Phillip Knipe and Philip Jennings Physics and Energy Studies, Murdoch University, Murdoch, WA 6150 Received 30 August Accepted 15 May 2005 ABSTRACT In recent years there has been growing concern about the potential link between various health effects, related to chronic low level exposure to electromagnetic radiation (EMR). While there is still no consensus in the scientific community about these effects, many authorities are urging a policy of prudent avoidance of unnecessary exposure to EMR. Regions of particular interest in the electromagnetic spectrum are the Extremely Low Frequency (ELF) Hz, Very Low Frequency (VLF) khz and Radio Frequency (RF) 100 khz 300 GHz bands. Remote area power supply (RAPS) systems are becoming increasingly prevalent. These systems tend to use renewable energy sources and their associated technology rather than the conventional diesel generator power supply systems. Recently some concerns have been raised about the levels of EMR being emitted from these new forms of technology. Some of the inverters transforming the generated direct current (DC) to alternating current (AC) have produced significant levels of EMR. These devices are often located close to living areas and therefore could increase the potential hazards to residents. While the main aim of these systems is to replace non-renewable energy sources and reduce greenhouse gas emissions, they should also be designed to minimise the doses of EMR to which users are exposed. This paper examines the various frequencies of EMR produced by typical remote area power supply systems (RAPS). The strengths of these fields are measured using various types of monitoring equipment and the health hazards are assessed. 34 The analysis of these results enables a qualitative assessment of the hazards associated with RAPS systems. This leads to a set of recommendations to system designers and health authorities on sensible measures to be adopted to minimise the potential risks from EMR to the users of RAPS systems. 1. INTRODUCTION Remote area power supply (RAPS) systems are usually specially designed to suit individual power supply needs. They are widely used in pastoral stations, remote indigenous communities and ecotourism facilities. RAPS systems generally contain a combination of renewable and fossil fuel generators plus inverters, power conditioners and batteries. The design of these systems depends on the location and load requirements. In many situations the solar panels and inverters are placed close to the dwelling (e.g. solar panels on the roof or inverters against the outer wall) due to design or engineering constraints. The placement of these components in close proximity to areas regularly occupied by the residents may result in them being exposed to elevated levels of electromagnetic radiation (EMR). The purpose of this study is to measure the EMR from RAPS system components in order to assess whether they pose any significant health risks to humans and if this occurs to make recommendations to minimise exposure to EMR. 2. COMPONENTS OF A RAPS SYSTEM There are three main categories of components in a RAPS system: 1 Energy generation equipment such as diesel generators, solar panels and wind turbines.

2 Figure 1 Renewable Energy Powered RAPS System [1] 2 Energy Storage Equipment usually batteries. 3 Power Conversion and Control Equipment controllers and inverters. The schematic design of a typical RAPS system is shown in Figure HEALTH EFFECTS For frequencies up to 100 khz the main interaction of electric and magnetic fields with tissue is the induction of currents. Exposure to frequencies above 100 khz could result in the induction of currents and absorption of sufficient energy to cause significant temperature rises [2]. The health effects for both bands can be divided into two main areas, recognised and uncertain. 1.1 Recognised Health Effects Exposure up to frequencies of 100 khz Measurements for this frequency range are conducted in the near field so the effects due to electric and magnetic fields will be considered separately. Electric Fields The main effects of electric fields are due to surface charges and small skin currents. The biological effects due to induced currents by electric fields range from changes in calcium metabolism or suppression of melatonin production for current densities of 1 to 10 ma/m 2 to demonstrated effects, such as changes in protein and DNA syntheses and in enzyme activity, evident visual and possible nervous effects. Also, the healing process of fractured bones can be accelerated or brought to a standstill for current densities of ma/ m 2. Extra systoles and ventricular fibrillation (heart dysfunction) can occur (acute health hazards) for levels above 1000 ma/m 2 [3]. 35 Magnetic Fields The biological effects due to induced current densities generated by magnetic flux densities from whole body exposure to sinusoidal homogeneous fields ranges from visual and nervous system effects for a magnetic flux of 5 50 mt to stimulation of excitable tissue causing possible health risks for a magnetic flux of mt. Extra systoles and ventricular fibrillation can occur (acute health hazards) at a flux density greater than 500 mt [3]. Exposure to frequencies from 100 khz up to 300GHz For frequencies of 100 khz up to 10 MHz, induced current densities higher than the threshold level of 100 ma/m 2 can excite muscles and nerves. For frequencies of 100 khz up to 10 GHz increases in the core body temperature and excessive localised tissue heating of more than one or two degrees Celsius over prolonged periods can cause adverse health effects such as heat exhaustion (headache, nausea, dizziness and heat stroke). The power density threshold for excessive heating of skin tissue and in tissue near the body surface is 500 Wm -2. For frequencies of 10 GHz up to 300 GHz the penetration depth past the skin layer is very small and only surface heating occurs. 3.2 Uncertain Health Effects Exposure up to 100 khz There are many investigations and studies being performed worldwide in an attempt to establish the health effects of low level exposure to these electromagnetic fields. The investigations can be separated into seven main areas: carcinogenicity in animals; carcinogenicity in adults;

3 carcinogenicity in children; non-cancer health effects in animals; non-cancer health effects in humans; environmental exposure; laboratory studies in vitro and mechanistic studies [4]. The most important and definitely the most topical health effect arising from the exposure to ELF and VLF electromagnetic fields is the possible development of childhood leukaemia. Recently Doll suggested that chronic exposure to magnetic fields above 4 mg are associated with a doubling of the risk of leukaemia in children under fifteen years of age [5]. Even though this level is not accepted as an international standard, we have used this figure as a benchmark in our measurements on RAPS systems under the precautionary principle. Exposure to EMR with Frequencies from 100 khz up to 300 GHz There are many investigations and studies being performed worldwide in an attempt to establish the biological and health effects of low level exposure to RF electromagnetic fields. The investigations can be separated into eleven main areas: cellular and molecular biology; biochemical changes; reproduction, growth and development; effects on the nervous system; behavioural effects; neuroendocrine effects; cardiovascular effects; effects on hematopoiesis and haematology; effects on immune response; auditory response, ocular effects [6]. Again the most important and definitely the most topical health effect arising from the exposure to RF electromagnetic fields is the possible development of cancer. Currently, the results of the residential and occupational epidemiological studies are the ones of most interest. 4.2 Measurement Methodology Inverter Measurement Methodology for the ELF and VLF Range There are a variety of inverters that are currently commercially available. They vary in design and operating size. Therefore, the manufacturer of the inverter should not be the most significant factor that affects the strength of the EMR emitted from these devices. Instead, properties such as topology, component selection, mechanical design and the level of screening incorporated into their design, will control emission levels. Since the actual design of the inverters may vary significantly from manufacturer to manufacturer, the location of the maximum and minimum EMR levels cannot be easily predicted. Hence, measurements were taken at nine locations across the surface of the front of the inverter (Figure 2) to establish the location of both the minimum and maximum EMR values for each distance from the inverter. Figure 2 Measurement Locations on Inverters for ELF and VLF Range Top LTC TMF TRC Middle MLS MCF MRS Bottom BLC BCF BRC 4. MEASUREMENTS 4.1 Measurement Equipment The following equipment was used for this work. ELF 50/60Hz Holaday Hi-3604 ELF / Power Frequency EMF Survey Meter VLF khz Holaday Hi-3603 VDT/ VLF Radiation Survey Meter RF 100 khz 2.06 GHz Protek RF Field Strength Analyser Where LTC refers to left top corner, MLS refers to left middle side of face and BLC refers to the bottom left corner. Measurements were taken at each of the nine positions for distances of 10, 20, 40, 80, and 160cm out from the outer surface of the inverter cabinet. The meter was aligned in each of the three axes to determine the axis with the maximum reading. This axis was then used for all further measurements of that particular inverter. This maximum axis was verified at other positions during the measurement. This measurement represents the root mean square (rms) value of the power radiating through the detector. This value does not represent the true 36

4 maximum value of the field strength but probably underestimates the true value to some degree. However, because of the squared relationship between field strength and distance from the inverter this underestimation is not significant enough to be considered an issue the further out the measurements are taken. Measurement Methodology for the RF Range (150kHz 2GHz) The two pieces of equipment selected for these measurements were the diesel generator (Generator 1) and the 20 kva inverter located in ACRELab at Murdoch University. The bandwidth chosen for the RF measurements is quite wide. It was not practical to measure the spectrum in one scan. For this reason the RF band was broken into 19 different sections. To establish if there were any significant RF emissions from the selected equipment, a background baseline was measured when the equipment was not operating. Measurement scans of the RF bandwidth were then completed when the equipment was operational. Because of the possibility of spurious results occurring during measurements, the bandwidth was scanned four separate times for each of the operational and non-operational situations for the generator and inverter. These multiple measurements allowed for signal averaging, reducing the impact of anomalous measurements. To eliminate the issue of transient features, data smoothing was carried out using the data smoothing tool available on the EasyPlot program The RF measurements, when the equipment was operating, and the background levels, were then plotted using EasyPlot. The difference between the two measurements was also plotted. 5. RESULTS 5.1 Inverters ELF and VLF The maximum ELF B-field levels measured for all the inverters ranged from 42 up to 327 mg. The strength of the B-fields from all of these inverters dropped below 4 mg at a distance of approximately 80cm from the inverters (Figure 3). Figure 3 Maximum ELF B-Field for Various Inverters 10 B-Field (mg) mg Inverter 4B Inverter 4A Inverter 4 Inverter 3 Inverter 2 Inverter Distance from Inverter (cm) The maximum ELF E-field level measured for all the inverters was well below the 5kVm -1 maximum specified by the current Australian Standards [7]. 37

5 Figure 4 Maximum VLF B-Field Levels for Various Inverters B-Field (mg) Distance from Inverter (cm) 4 mg Inverter 4 Inverter 4B Inverter 4A Inverter 2 Inverter 1 The maximum VLF H-field level measured for all the inverters was 996mA/m, which is well below the recommended ICNIRP limit of 2.43 to 5 A/m for the 3 to 300 khz band [2]. The maximum VLF E-field level measured for all the inverters was 0.96 V/m, which is well below the recommended ICNIRP limit of 87 Vm -1 for the 3 to 300 khz bandwidth [2]. RF Levels There were some measurable RF EMR emissions, in the 0.15 to 77.5 MHz band above background from the inverter, when it was operating. There were no significant RF EMR emissions measured above background in the 77.5 to 2000 MHz band. 5.2 Generators ELF and VLF The maximum ELF B-field levels measured for all the generators ranged from 42 up to 327 mg. The strength of the B-fields from these inverters all dropped below 4 mg at a distance of approximately 80cm from the generators (Figure 5). The maximum ELF E-field level measured was 7 V/m on the LHS at 10cm. This is well below the 5kVm -1 limit specified in the Australian Standards. The maximum VLF H-field level measured was 35 ma/m on the LHS at 10cm. This is below the recommended ICNIRP limit of 2.43 to 5 A/m for the 3 to 300 khz bandwidth. 38 The maximum VLF E-field level measured was 0.12 V/m at 10cm on the LHS. This is well below the 87 Vm -1 ICNIRP limit. RF Levels There were no significant RF EMR levels measured above background from the generators when they were operating. This result is not surprising because the diesel generator is commercially produced and would be required to adhere to electromagnetic compatibility (EMC) requirements. 5.3 Battery Bank The maximum ELF B-field level measured was 9.24 mg at the middle RC location at 10cm. The measured B-field for all locations dropped below the 4 mg mark at a distance of 40cm from the outer surface of the cabinet. The maximum E-field measured was 26.6 V/m at the middle RCM location at a distance 40cm. 6. CONCLUSIONS Some RAPS equipment does emit EMR at levels that may cause concern (B>4mG). However provided these devices are situated at least 1.6 metres from regularly occupied areas there should be no cause for concern. The major sources of EMR emissions are generators, inverters and battery banks. The main area that could cause concern is ELF magnetic fields emitted by this equipment

6 Figure 5 Generator ELF B-Field Strength Vs Distance B-Field (mg) Distance from Generator (cm) 4 mg Gen 1 Charging Batt - RHS Gen 1 Charging Batt - LHS Gen 1 Full Load - RHS Gen 1 Full Load - LHS Gen 2 Load - RHS Gen 2 Load LHS Gen 2 No Load - RHS Some inverters produce detectable RF emissions 7. RECOMMENDATIONS Until research determines whether there are health hazards associated with chronic exposure to low level EMR in the ELF, VLF and RF bands a precautionary approach is advisable. Simple steps like the appropriate placement of the various components of RAPS systems ie at distances greater than 160cm for inverters and generators from regularly occupied areas are recommended. The elevated levels of RF EMR emitted by RAPS inverters in the 0.15 to 77.5 MHz bandwidth should be investigated further. In particular, the reduction of these elevated levels may be achieved using technical solutions of minimal cost. This is justified because there is a possibility of health hazards being associated with long term exposure to low level RF EMR. As the technology of RAPS systems develops, the levels of EMR from new generations of RAPS system components should be checked. EMR limits and testing procedures for inverters and generators should therefore be included in the standards that are being developed for RAPS systems. 39 Acknowledgements The authors wish to thank the staff of ACRELab (now ResLab) for providing access to their equipment for this project. This work formed part of Phillip Knipe s MPhil thesis at Murdoch University. REFERENCES 1. Wilmot, N., Remote area power supply (RAPS) information sheet. 1994, Murdoch University Energy Research Institute (MUERI): Perth. 2. UNEP/WHO/ICNIRP, Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300 GHz). Health Physics, (4): p UNEP/WHO/IRPA, Environmental Health Criteria 69: Magnetic fields. Environmental Health Criteria , Geneva: World Health Organization. 4. NAS, Possible health effects of exposure to residential electric and magnetic fields. 1997, Washington DC: National Academy Press. 5. AGNIR, ELF electromagnetic fields and the risk of cancer. Doc NRPB, (1): p Michaelson, S.M. and E.C. Elson, Interation of nonmodulatred and pulse modulated radiofrequency fields with living matter : Experimental results, in Handbook of biological effects of electromagnetic fields, C. Polk and E. Postow, Editors. 1996, CRC Press Inc. p NHMRC, Health Series No 30: Interim guidelines on limits of exposure to 50/60 Hz electric and magnetic fields. Health Series No

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